Mould parts, inserts or dies for forming articles are used in many different processes, such as injection moulding, blow moulding, hot pressing, stamping, thermoforming, embossing, coining or printing, etc. The mould part can be manufactured in many different ways. It can be milled from a solid member of metal or other material. Another method is to manufacture the mould by electroforming. A master with the shape of the article to be manufactured by the mould is provided. The master can be of an electrically conductive material or be provided with a surface of an electrically conductive material. A relatively thick “mould layer” of metal is electroplated on the surface of the master. The master is then removed, e.g. by dissolving, and the electrodeposited metal layer can be used as a mould part for forming articles with the same shape as the master. The metal mould layer can also be deposited by autocatalytic plating, also known as electroless plating. Electroless plating is a plating process including the step of deposition but without the application of current. The process is a chemical reaction and is autocatalytic. The master can be made of aluminium which is easy to machine to the desired form and easy to remove by etching. Being easy to plate and hard, i.e. a good wear resisting material, nickel is often chosen for mould parts made by electroforming. Cobalt has very similar characteristics and is also suitable. Normally, the mould layer is backed with another material, e.g. further metal plating to provide sufficient rigidity and thermal conductivity.
US 2003/0090030 A1 discloses the machining of a master in aluminium and the electroplating of same in a nickel bath to provide a mould part.
Furthermore, it is known to remove the aluminium master by dissolving the aluminium in an alkaline solution. Aluminium can be dissolved by an alkaline solution comprising hydroxide such as NaOH without dissolving any nickel or cobalt. Thus, a perfect surface representing the reverse image of the master is left when the aluminium has been dissolved. However, this requires that the master is made of completely pure aluminium. Pure aluminium is not suitable for chip-producing processing as long chips are produced. Aluminium alloys suitable for machining comprise alloy elements, such as Cu, Mn, Si, Mg, Zn, Sn, to improve different properties, e.g. machining properties. However, several of these alloying elements form oxides which are not easily removed by the alkaline solution. In order to remove these oxides or other chemical combinations comprising these alloying elements, more aggressive solutions must be used. In most cases, these aggressive solutions will etch or dissolve some of the nickel or cobalt resulting in a mould part with a surface which is not a perfect reverse image of the master. In addition, environmental considerations must to be taken into account when using aggressive solutions.
Alternatively, a zinc alloy can be used as a material for the master. Like aluminium, zinc can be dissolved in an alkaline solution.